Field of Invention
The present invention relates to the removal of contaminants, particularly particulates, from liquids by filtration.
Prior Art
One of the most significant drawbacks to filtration and infiltration systems used for treating contaminated water is premature clogging of the surface due to buildup of solids. In industrial and drinking water treatment, filters are backwashed periodically to restore hydraulic flow, or other mechanical methods may be used (U.S. Pat. No. 7,163,630). This is not possible in typical stormwater filtration and infiltration devices which operate passively (without power) under gravity filtration. Many stormwater devices are therefore part of systems that incorporate some combination of filter media, hydrodynamic sediment removal, oil and grease removal, or screening to remove floatables and particles that could prematurely blind the filter media. A typical sand filter system includes a pretreatment or sedimentation chamber that prolongs filter media life by removing floatables and heavier suspended solids. Infiltration treatment systems are normally combined with pretreatment such as grass strips or swales, or sediment basins to prevent premature clogging. Dual layer filters in which a coarse grain size media layer is on top of a finer grain size layer is another technique used to prolong the hydraulic life of filters (U.S. Pat. Nos. 7,045,067; 5,281,332; 3,876,546). Ultra-urban filter devices incorporate vertical flows or use relatively coarser media or media in pellet form to prevent premature clogging of the media (U.S. Pat. Nos. 7,419,591; 6,649,048; 5,707,527). In general, the trade off is between using small grain size filter media which improves treatment but results in quicker clogging, and using coarser grain size media which provides poorer treatment but prolongs hydraulic capacity.
In industrial applications, high contact area filter elements are used together with backwashing to prolong the service life of pressurized filtration systems. These elements, called “filter candles”, are typically tubular or cylindrical and consist of slotted or perforated material, such as wire mesh or wedge wire, of various slot or opening sizes to suit the application (U.S. Pat. No. 5,474,586). Increasing the filtration contact area using a high contact area device has also been proposed for molten polymer filtration (U.S. Pat. No. 3,044,628). Screen filters have recently been proposed in wastewater and stormwater treatment applications (U.S. Pat. Nos. 3,875,055, 4,235,724; 5,492,635; 6,337,025 B1; 6,533,941). Cartridge type filter devices used to treat environmental flows, including stormwater runoff, can also utilize screens both to filter particulates and as a means of directing flows in and out of the individual cartridges (U.S. Pat. Nos. 6,649,048; 5,024,771).
The prior art shows that clogging of granular media filters is a major concern in terms of service life and maintenance costs. This is particularly true for “passive” or gravity flow filters commonly used for stormwater treatment because backwashing is often impractical. It is an object of this invention to use a specially shaped, high surface area device to improve the service life of granular media filtration and infiltration media used for passing or treating contaminated water such as stormwater runoff. The improved service life is achieved by using the device to create a layer of granular media (a filter layer) that has increased area available to pass the contaminated water flow. Increasing the area available to pass flow reduces the rate of clogging of the filter and therefore improves the service life. It is also an object of this invention to provide some treatment of stormwater runoff by removing pollutants and particles and to promote infiltration by increasing the volume capture of the contaminated water, particularly stormwater runoff.